Gardasil is designed to protect against HPV types 6, 11, 16, and 18. HPV 16 and 18 cause an estimated 70% of cervical cancer cases, and HPV 6 and 11 account for some 90% of cases of genital warts, which can lead to abnormal Pap tests and low-grade cervical abnormalities.

Merck estimates the vaccine could reduce deaths worldwide from cervical cancer by two-thirds. The expected price (about $300 to $500 for three shots over 6 months), however, could limit its use.

The CDC's Advisory Committee on Immunization Practices has recommended that 11- and 12-year-old girls be routinely vaccinated with Gardasil and that girls as young as 9 can receive the shots, at their physician's discretion.